Thin and light weight light-emitters with high efficiency have recently been required more and more in order to achieve thinning of flat panel displays, compacting and thinning of electronic equipments with liquid crystal displays, improving of shape-design flexibility for sign lights and luminaires, or the like. The organic EL elements are able to respond to such requirements, and the elements have been focused on.
C. W. Tang, et al. of Eastman Kodak company first disclosed that an organic EL element emitted light at a low voltage through the structure comprising electrodes and two thin films layered between the electrodes (Appl. Phys. Lett., 51,12,913(1978)). Since then, the industrial world has paid attention to various advantages of the organic EL elements, such as high intensity light emission (e.g., 100-10000 cd/m2) in response to a low voltage (e.g., several voltages), multi-color light emission by combination of materials constituting the light emitting layers (active layers), availability as extremely thin plane light emitters, and so on, and has examined organic EL elements having various improved thin film structures.
A fundamental structure of an organic EL element consists of an anode, an organic light emitting layer and a cathode. But there are various structures such as: structure comprising anode, hole transport layer, organic light emitting layer, electron transport layer and cathode; structure comprising anode, hole injection layer, hole transport layer, organic light emitting layer, electron transport layer and cathode; structure comprising anode, hole injection layer, organic light emitting layer, electron transport layer, electron injection layer and cathode; structure comprising anode, hole injection layer, organic light emitting layer, electron injection layer and cathode; and so on. It is generally known that injection characteristic of holes and electrons from electrodes (anode and cathode) to an organic light emitting layer is improved by such multilayer structures, and also light emission characteristic is improved by that the organic light emitting layer is defined as a recombination region of holes and electrons. In addition, the light emission characteristics of an organic EL element is drastically improved as compared to those days through advancement of materials for an organic EL element in recent years. Also the lifetime, but there was apprehension at first, is raised to a comparatively high level. Accordingly, organic EL elements have started to be put to practical use in various applications (see Japanese Patent Application Publication Number H09-139288).
However, further improvement in efficiency and lifetime of an organic EL element has been required, and therefore improvement in the device structure is necessary in addition to that of the materials. Recombination of holes and electrons outside a light emitting layer gives the organic EL element a reduction factor of the efficiency and lifetime. There is energy loss that the recombination energy cannot contribute to emission of light. The cause of the loss is that capability for electron injection from the cathode side to the organic light emitting layer and/or hole injection from the anode side to the organic light emitting layer is low and also that valance and injection quantity of holes and electrons into the organic light emitting layer are not optimum. Consequently, in order to provide an organic EL element with high efficiency and long lifetime, it is necessary to raise the performance of an electric charge (holes and/or electrons) injection into the organic light emitting layer, and also to optimize the injection valance.
What is needed, therefore, is that holes and electrons are efficiently recombined only in the organic light emitting layer.